According to the Gang of Four, the intent of the Adapter pattern is to
Convert the interface of a class into another interface that the clients expect. Adapter lets classes work together that could not otherwise because of incompatible interfaces.[1]
[1] Gamma, E., Helm, R., Johnson, R., Vlissides, J., Design Patterns: Elements of Reusable Object-Oriented Software, Reading, Mass.: Addison-Wesley, 1995, p. 185.
Basically, this is saying that we need a way to create a new interface for an object that does the right stuff but has the wrong interface.